Apparatus and method for detecting far end speech

ABSTRACT

Method and apparatus for detecting far end speech in a telephone apparatus. A speaker ( 14 ) produces far end sound from a far end signal, when received by the telephone apparatus. A microphone ( 12 ) generates a near end signal from near end sound, for transmission from the telephone apparatus. A hybrid ( 24 ) which passes the near and far end signals from an external two wire link ( 30 ) to and from the speaker and microphone, on respective first and second paths ( 1, 2 ) of a four wire link ( 28 ). Muting of the near end signal is effected when signal derived from the first path ( 1 ) of the four wire link ( 28 ) is detected as being representative of a condition of existence of far end signal. Signal determined as representing sidetone signal deriving from the near end signal from the speaker ( 14 ), and appearing in the signal on the first path ( 1 ) of the four wire link ( 28 ), pursuant to traverse of part of the near end signal through the hybrid, is subtracted from the signal derived from the first path ( 1 ) of the four wire link ( 28 ). Existence of the condition of existence of far end signal is then determined on the basis of the result of the subtraction.

This is a National Stage of International Application PCT/AU00/00375,with an international filing date of Apr. 28, 2000, and the completedisclosure of which is incorporated into this application by reference.

BACKGROUND OF THE INVENTION

This invention relates to apparatus and method for detecting far endspeech.

In telephone systems designed to operate with a half duplexcommunications system, the outgoing speech signal from the near endtelephone apparatus is muted so as not to be transmitted to the far enduser when the far end user is speaking. Otherwise, the far end speech ismuted, at the near end telephone apparatus. This may be effected bydetecting at the near end a condition where the received far end speechsignal is of significant magnitude, and, if so, effecting muting at thenear end of the outgoing near end speech signal, the far end speechsignal at the near end telephone apparatus otherwise being muted.

Telephone apparatus for implementing the above generally employs,internally, four wire signal transmission. That is to say, the signalsfrom and to the telephone microphone and loudspeaker respectively are onseparate communications paths, this being necessary to enable near endand far end signals to be distinguished as necessary for performing halfduplex operation. The telephone apparatus may thus incorporate a hybridthe purpose of which is to couple the two and four wire signal paths. Atthe microphone and loudspeaker, the signals are in analogue form.Transmission over the communications link between the near and far endtelephone apparatuses will normally be effected by analog signals over atwo wire link, such as a Public Subscriber Telephone Network (PSTN).Internally, each telephone apparatus may incorporate some means forconverting to and from digital and analogue signal formats.

If the impedance characteristics of the hybrid provide exact signalmatching, outgoing near end signal, from the near end microphone, isdirected entirely to the two wire link. However, if there is imperfectmatching, some part of the near end signal from the near end microphonewill pass through the hybrid to be mixed with incoming far end signal onthe signal path to the speaker of the near end telephone apparatus.Practically, it is impossible to ensure exact matching, so there isalways some returned signal of this kind, called “sidetone”.

The existence of sidetone signal presents a particular problem in halfduplex signal transmission, because it can interfere with the ability todetect, at the users' telephones, when the far end user is or is notspeaking, with the consequence that conditions requiring muting ornon-muting of speech signals are not properly detected. A particularproblem of this nature occurs where the near end telephone is in anenvironment where there is an acoustically noisy background. In thiscase, the side-tone signal may be of significant magnitude, such thatdetection of a condition where the far end user is not speaking is notproperly effected because there is sufficient sidetone signal mixed withthe incoming far end speech signal to initiate muting of the outgoingnear end speech signal, regardless of whether or not the far end user isspeaking, it being impossible to distinguish the sidetone from the farend user's speech signal. That is, at the near end telephone, side-tonesignal is detected and interpreted as indicating a condition where thefar end user is speaking, so that the far end signal is, perhapsconstantly, muted at the near end user's telephone. Of course, in thiscircumstance, communication between the parties is interfered with andmay be rendered impossible.

SUMMARY OF THE INVENTION

In one aspect, the invention provides detection apparatus for detectingfar end speech in a telephone apparatus having a first transducer forreproducing far end sound from a far end signal, when received by thetelephone apparatus, and a second transducer for generating a near endsignal from near end sound, for transmission from the telephoneapparatus, and in which the near end signal is muted for transmissionfrom the telephone apparatus in the case where it is intended that auser of the telephone apparatus should listen to the reproduced far endsound, and a hybrid for passing the near and far end signals from anexternal two wire link to and from the first and second transducers onrespective first and second paths a four wire link, the detectionapparatus being in use responsive to effect said muting of the near endsignal when signal derived from said first path of said four wire linkis detected as being representative of a condition of existence of farend signal, and subtracting, from the signal derived from said firstpath of said four wire link, signal determined as representing sidetonesignal deriving from the near end signal from the second transducer andappearing in the signal on said first path of said four wire linkpursuant to traverse of at least part of the near end signal through thehybrid, existence of said condition being determined on the basis of theresult of the subtraction of the signal determined as representingsidetone signal from the signal on said first path of said four wirelink.

By this, the signal on the basis of which detection of existence of saidcondition is made may be made more representative of the far end signal.Thereby, reliability of operation of the telephone apparatus may beimproved.

In embodiments of the invention the signal determined as representingsidetone signal is, in use, derived from said second path of said fourwire link.

The detection apparatus may be implemented as hardware, but it may moreconveniently be implemented wholly or partly as software. In a softwareimplementation, the software ma %, implement a subtracter in useeffecting said subtracting.

The subtracting may be effected by generating, from the signal on saidsecond path of said four wire link, a first relatively fast trackingenvelope of the signal on said second path of said four wire link, andsubtracting a proportion of the first envelope from the signal derivedfrom the first path of the four wire link. The first envelope may, inthe case where a condition of existence of far end speech was previouslydetected, be the envelope of the muted near end signal, and otherwise isthe envelope of the non-muted near end signal.

The detection apparatus may be effective in use to generate a detectionsignal, indicative of said condition, from the long term envelope ofsignal representing the result of said subtraction, and the short termenvelope of that signal. The detection signal may be representative of acondition where no far end speech is present when if a condition ofdetection of far end speech prevails, a short term envelope of saidsignal representing the result of said subtraction is less than a meanbased on the product of a noise long term envelope and the long termenvelope of signal representing the result of said subtraction, andrepresentative of a condition where far end speech is present when, if acondition of detection of far end speech does not prevail, a very shortterm envelope of said signal representing the result of said subtractionis greater than said mean based on the product of a noise long termenvelope and the long term envelope of signal representing the result ofsaid subtraction.

The detection apparatus may in use apply the detection signal toincrementally adjust the magnitude of a hold signal, which hold signalwhen it reaches a threshold level controls the telephone apparatus toeffect said muting of the near end signal, but otherwise which socontrols the telephone apparatus that the near end signal is not muted.

In a particular form, the detection apparatus has:

-   -   a near very short term envelope generator for generating a near        very short term envelope signal representing the very short term        envelope of the near end sound: and    -   a subtracter for generating a difference signal by subtracting a        proportion of a muted component of the near very short term        envelope signal from the far end signal, in case of existence of        a detected condition of far end speech, or otherwise subtracting        a proportion of a non-muted component of the near very short        term envelope signal from the far end signal;    -   an output of the apparatus in use being indicative of said        condition of the existence of far end speech when said        difference signal exceeds a predetermined magnitude.

The invention also provides a method of detecting far end speech in atelephone apparatus having first transducer for reproducing far endsound from a far end signal, when received by the telephone apparatus, asecond transducer for generating a near end signal from near end sound,for transmission from the telephone apparatus, and in which the near endsignal is muted for transmission from the telephone apparatus under inthe case where it is intended that a user of the telephone apparatusshould listen to the reproduced far end sound, and a hybrid for passingthe near and far end signals from an external two wire link, to and fromthe first and second transducers on respective first and second paths afour wire link, the method comprising effecting said muting of the nearend signal when signal derived from said first path of said four wirelink is detected as being representative of a condition of existence offar end signal, and subtracting, from the signal derived from said firstpath of said four wire link, signal determined as representing sidetonesignal deriving from the near end signal from the second transducer andappearing in the signal on said first path of said four wire linkpursuant to traverse of at least part of near the end signal through thehybrid, existence of said condition being determined on the basis of theresult of the subtraction of the signal determined as representingsidetone signal from the signal on said first path of said four wirelink.

By this, the signal on the basis of which detection of existence of saidcondition is made may be made more representative of the far end signal.Thereby, reliability of operation of the telephone apparatus may beimproved.

In embodiments of the method of invention the signal determined asrepresenting sidetone signal is derived from said second part of saidfour wire link.

The subtracting may be effected be generating, from the signal on saidsecond path of said four wire link, a first relatively fast trackingenvelope of the signal on said second path of said tour wire link, andsubtracting a proportion of the first envelope from the signal derivedfrom the first path of the four wire link. The first envelope my, in thecase where a condition of existence of far end speech was previouslydetected, be of the muted near end signal, and otherwise be of thenon-muted near end signal.

The method may further comprise generating a detection signal,indicative of said condition, from the long term envelope of signalrepresenting the result of said subtraction, and the short term envelopeof that signal. The detection signal may be representative of acondition where no far end speech is present when, if a condition ofdetection of far end speech prevails, a short term envelope of saidsignal representing the result of said subtraction is less than a meanbased on the product of a noise long term envelope and the long termenvelope of signal representing the result of said subtraction andrepresentative of a condition where far end speech is present when, if acondition of detection of far end speech does not prevail, a very shortterm envelope of said signal representing the result of said subtractionis greater than said mean based on the product of a noise long termenvelope and the long term envelope of signal representing the result ofsaid subtraction.

The detection signal may be applied to incrementally adjust themagnitude of a hold signal, which hold signal when it reaches athreshold level controls the telephone apparatus to effect said mutingof the near end signal, but otherwise which so controls the telephoneapparatus that the near end signal is not muted.

In a particular form, the method further comprises:

-   -   generating a near very short term envelope signal representing        the very short term envelope of the near end sound;    -   generating a difference signal by subtracting a proportion of a        muted component of the S neai very short term envelope signal        from the far end signal, in case of existence of a detected        condition of far end speech, or otherwise subtracting a        proportion of a non-muted component of the near very short term        envelope signal from the far end signal; and    -   generating an output indicative of said condition of the        existence of far end speech when said difference signal exceeds        a predetermined magnitude.

The detection apparatus and method may repetitively execute steps ofgenerating near very short term envelope signal, and generating saiddifference signal.

The invention also provides a telephone apparatus fitted with thedetection apparatus of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of example with reference tothe accompanying drawings in which:

FIG. 1 is a diagram of a half duplex telephone apparatus constructed inaccordance with the invention;

FIGS. 2 (a), 2 (b), 2(c) and 2 (d) illustrate signal waveforms relevantto the operation of the telephone apparatus of FIG. 1:

FIG. 3 is a diagram of steps executed by a near end speech detector, inone embodiment of the invention;

FIG. 4 is a diagram of steps executed by a half duplex switching device,in one embodiment of the invention; and

FIG. 5 is a diagram of steps executed by a far speech detector, in oneembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a telephone apparatus 10, intended for half-duplexoperation. This has a microphone 12 for generating a near end electricalsignal from speech sound from a user, and a speaker 14 for reproducingsound from an applied far end electrical signal.

The apparatus 10 also incorporates a control device 16, two codecs 20,22 and a hybrid 24. The control device 16 operates on digitised signals,in a manner described later, to effect half duplex operation ofcommunications to and from the telephone apparatus.

The codec 20 is positioned between the control device 16 and themicrophone 12, and serves to digitise near end analogue signal from themicrophone for application to the control device 16. The codec 20 alsoacts to convert digitised far end signal received from the controldevice 16 to analogue form for application to the speaker 14.

Codec 22 is positioned between the control device 16 and the hybrid 24.It serves to convert digital far end signal from the control device 16to analogue form, for application to the hybrid 24, and to convertanalogue far end signal from the hybrid 24 to digital form forapplication to the control device 16.

The signal link 28 between the hybrid 24, control device 16, codecs20,22, microphone 12 and speaker 14 is a four wire link. That is to say,incoming far end signal travels on a path “1” of the link 28, formed oflink parts 28 a, 28 b, and 28 c from the hybrid 24 to the speaker 14,being separate from the path “2” of link 28, formed of link parts 28 d,28 e and 28 f on which signal travels from the microphone 12 to thehybrid 24. The hybrid 24 serves to connect the four wire signal paths oflink 28 to the two wire link provided by a public subscriber telephonenetwork 30, via which communication between the telephone apparatus 10and another similar apparatus is established.

In the case where the hybrid 24 provides perfect matching as between thefour wire signal link in the telephone apparatus 10, and the two wirePSTN, signals to and from the telephone apparatus 10 travel to and fromthe telephone apparatus with outgoing signal from the codec 22 goingwholly to the PSTN, and incoming signal from the PSTN going wholly tothe codec 22. If, however, there is any mis-match provided by the hybrid24, some part of the outgoing near end signal from the codec 22 will bepass through the hybrid 24 and be returned to the codec 22 as sidetone.Practically, there will always be some returned sidetone, since it isimpossible to reliably ensure that perfect matching occurs.

The function of the control device is, responsive to existence ofdigitised far end signal at the respective input 16 c thereto, to mutethe digitised near end speech signal applied at the relevant input 16 athereof, so that only a significantly reduced near end signal is passedto the corresponding output 16 b thereof, and thence to the PSTN via thecodec 22 and hybrid 24. When far end signal is not detected at input 16c to control device 16, incoming far end signal applied to input 16 c ismuted so as only a significantly reduced signal appears at therespective output 16 d of the control device 16.

The satisfactory operation of a simple arrangement where muting isdependent on the existence of signal at the input 16 c to the controldevice 16 is dependent upon the signal at that input accuratelyrepresenting the far end speech. However, the applied signal will notcomprise only the far end speech signal if there is significant sidetonesignal present. Particularly if the telephone apparatus is situated in anoisy environment, the applied signal may, depending on the degree ofperfection of matching provided by the hybrid 24, include a significantsidetone component, and this may give rise to faulty operation. That is,if the sidetone signal is significant, the control device may interpretthis as indicating the presence of far end speech signal where no suchsignal exists. Thus the outgoing speech signal may be muted, possiblyconstantly, but in any event for times which render difficult orimpossible ordinary voice communication between the users.

In order to improve the resistance offered by the control circuit 16 tounintended muting arising in the above way, the control device 16 isarranged to discriminate between far end speech signal and sidetone, andto be responsive, to a substantial extent, only to the far end speechsignal.

FIG. 2( a) is a graphical representation of a typical speech signal 40.The signal 40 contains components of various frequencies, but as shownin FIG. 2( b) the fast tracking envelope 4′ of the signal 40 exhibits apulse-like character. On the other hand, a slow tracking envelope 44 ofthe speech signal 40 will appear as in FIG. 2( c). While this slowtracking envelope 44 exhibits timewise variation, that variation issubstantially less. FIG. 2( d) shows the envelopes 42 and 44 overlaid.The technique employed in this embodiment of the invention involvesresponding to conditions where the amplitude of the fast envelopeexceeds that of the slow tracking envelope by a determined amount. Inparticular, conditions are detected where the prevailing positiveamplitude of the fast tracking envelope is greater by a predeterminedmargin than that of the slow tracking envelope, and where the prevailingnegative amplitude of the fast tracking envelope is less by apredetermined margin than that of the slow tracking envelope. Themargins in question may be selected as appropriate, being represented bylines 46, 48 in FIG. 2( d).

The above is effected under software control within the control device16. There are three basic parts to this, namely half duplex switching,far speech detection and near speech detection. Process steps inimplementing each of these functions are illustrated in FIGS. 3 to 5.The steps illustrated are executed cyclically and repetitively, as nowdescribed.

In the following description. “near end speech signal” refers to thesignal on link part 28 d of link path 2 of four wire link 28, as appliedto the device 16, from the microphone 12, and “far end speech signal”refers to the signal on link part 28 b of link path 1, as applied to thedevice 16, or as subsequently processed. Of course, at link part 28 b,to some extent within device 16, and even when the signal is passed fromdevice 16 to speaker 14, the far end speech signal may represent onlyone component of the signal passing to, within, or from device 16, sincesome sidetone component may be included. Similarly, there may forexample be associated noise components.

Referring to FIG. 4, near speech detection is executed first, at step401. This is used, as described later with reference to FIG. 3, tocalculate the near very-short-term envelope, for use in far speechdetection.

Next, a far speech detection function is called, at step 402. Thisfunction, described later with reference to FIG. 5, is used to detect iffar-end speech is present.

At the next step, 403, if far-end speech is present, then signals fromthe near end are muted, at step 404. Otherwise, if far speech is notpresent, signals form the far end are muted, at step 405. Generally,muting is effected so as to greatly reduce, but not entirely eliminate,the affected signal.

Near Speech Detection

The input to the near speech detector 300 (FIG. 3) is a near inputsample 301. Near end samples 301 are obtained repetitively from the nearend speech signal, the latter being sampled internally of the controldevice 16 at a received at a rate of 8000 Hz, so that a fresh sample isobtained every 125 microseconds. The near input samples are applied to anear gain calculator 302, to limit digital clipping. Following this,pre-emphasis is applied, at step 303, to emphasise the higherfrequencies, which assists in speech detection. The output from step 303is the calculated magnitude of the pre-emphasised near signal.

The near very short-term envelope detector 304 operates on the outputfrom step 303, and operates in the same way as the far very short-termenvelope detector 505 described below with reference to FIG. 5, exceptthat it operates on the near end signal to provide at its output thenear very short term envelope.

Far Speech Detector

The input to the far speech detector 500 (FIG. 5) is the far inputsample 501. Far input samples 501 are obtained repetitively from the farend speech signal, the latter being sampled internally of the controldevice at a rate of 8000 Hz, so that a fresh sample is obtained every125 microseconds. The samples are applied to a far gain calculator 502to limit digital clipping. Pre-emphasis is effected at step 503 toemphasise the higher frequencies, which assists in speech detection.Output from step 503 is the magnitude of the pre-emphasised far signal.

Far sidetone subtraction is employed, at step 504, to remove thesidetone caused by the near end signal on the far side. The operation isas follows:

-   -   if far speech is present, then a proportion of the muted        component of near very short-term envelope, generated at step        304 is subtracted from the far signal magnitude.

Otherwise, a proportion of the non-muted component of near veryshort-term envelope generated at step 304 is subtracted from the farsignal magnitude.

The proportion of signal subtracted is preselected, for example beingdetermined by user trials to ensure near end noise does not result infar-end speech being detected, but that detection of actual far-endspeech is sufficiently sensitive.

A far very short term envelope detector 505 generates a very fastreacting envelope on the far end signal. The operation is as follows:

-   -   if the far signal magnitude is greater than the far very        short-term envelope, then the latter envelope is updated by        summing a proportion of the signal magnitude, with an inverse        proportion of the existing very short-term envelope. Otherwise,        a “capacitive” decay is applied to the existing very short-term        envelope.

The proportion of signal magnitude and the speed of capacitive decayare, relatively, very fast. This envelope is intended only to smooth thesignal marginally.

A far short-term envelope detector 506 operates in the same way as thevery short-term envelope detector, except that it employs a slower risetime (ie a smaller proportion of the signal magnitude is used) andslower capacitive decay. This envelope is intended to accurately tracethe speech envelope.

Far speech long-term envelope detection is effected at step 507 in twomodes: tracking mode and drift mode. Tracking mode operates when farspeech is indicated, at step 515, as having been present (ie as havingbeen detected during a previous cycle of operation), otherwise driftmode operates.

In tracking mode, the far speech long-term envelope detector operates inthe same way the far short-term envelope detector, but with even slowerrise time and capacitive decay.

In drift mode, the far speech long-term envelope detector has acapacitive decay similar to that mentioned above) with offset added.

Far noise long-term envelope detection is effected at step 508 in thesame way as far speech long-term envelope detection, except that, inthis case, tracking mode is active when no far speech is present, anddrift mode is active when far speech is present.

The far geometric mean of the far speech long-term envelope and thenoise long-term envelope is calculated at step 509, this being thesquare root of the product of the speech long-term envelope and thenoise long-term envelope.

Far speech detection occurs, at step 510 as follows:

-   -   if far speech was detected previously (step 515) and the far        short-term envelope (step 506) is less than the aforementioned        far geometric mean multiplied by a hysteresis factor, then        speech is taken as not being detected. Otherwise, if far speech        was not detected decrement hold on speech being present. If        speech is no longer to be held, the speech is set to not being        present.

Far speech detection is thus effected at this point by a methodcorresponding to that described above with reference to FIG. 2, the longterm envelope 44 however being adjusted to take account of noise andhysteresis (and so represented by the described geometric meanmultiplied by a hysteresis factor) and the far short term envelope beingadjusted to a “very short term” envelope in the case where there is nocondition of previous detection of speech.

Speech detection hold is effected at step 511, this operating asfollows:

-   -   if far speech was detected, that condition is set at step 512        and, at step 511, a hold on speech present condition is set at        step 515, the latter condition being set as a desired numerical        value;    -   otherwise, if far speech was not detected, the hold on speech        being present numerical value (step 511) is decremented. If a        condition of speech no longer held is reached (ie the numerical        value held at step 511 is decremented to a predetermined value),        then a condition of speech not being present is set at step 512.

The hold on speech function described is used to avoid rapid switchingbetween conditions where speech is indicated as being present and notbeing present.

Generally, in the described control circuit, speech is detected/notdetected on repetitive cycles of operation of the device, but speech maybe regarded as being present even if speech was not detected.

The described arrangement has been advanced merely by way of explanationand many modifications may be made thereto without departing from thespirit and scope of the invention which includes every novel feature andcombination of novel features herein disclosed. For example, while aspecific method based on envelope detection is described for using, thedifference signal resulting from subtraction of the sidetone signal fromthe part 28 b of link 28 for determining whether far end speech exists,in principle any, known technique for performing this detection based oninput directly of signal from the link part 28 b may be adapted byproviding that this difference signal is used as input instead of thesignal on link part 28 b.

Throughout this specification, unless the context requires otherwise,the word “comprise”, and variations such as “comprises” and“comprising”, will be understood to imply the inclusion of a statedinteger or step or group of integers or steps but not the exclusion ofany other integer or step or group of integers or steps.

1. Detection apparatus for detecting far end speech in a telephoneapparatus having a first transducer for reproducing far end sound from afar end signal, when received by the telephone apparatus, and a secondtransducer for generating a near end signal from near end sound, fortransmission from the telephone apparatus, and in which the near endsignal is muted for transmission from the telephone apparatus in thecase where it is intended that a user of the telephone apparatus shouldlisten to the reproduced far end sound, and an hybrid for passing thenear and far end signals from an external two wire link to and from thefirst and second transducers on respective first and second paths of afour wire link, the detection apparatus being in use responsive toeffect said muting of the near end signal when signal derived from saidfirst path of said four wire link is detected as being representative ofa condition of existence of a far end signal, and subtracting, from thesignal derived from said first path of said four wire link, signaldetermined as representing sidetone signal deriving from the near endsignal from the second transducer and appearing in the signal on saidfirst path of said four wire link pursuant to traverse of at least partof the near end signal through the hybrid, existence of said conditionbeing determined on the basis of the result of the subtraction of thesignal determined as representing sidetone signal from the signal onsaid first path of said four wire link; wherein the signal determined asrepresenting sidetone signal is, in use, derived from said second pathof said four wire link; wherein said subtracting is effected bygenerating, from the signal on said second path of said four wire link,a first relatively fast tracing envelope of the signal on said secondpath of said four wire link, and subtracting a proportion of the firstenvelope from the signal derived from the first path of the four wirelink; and wherein the first envelope is, in the case where a conditionof existence of far end speech was previously detected, the envelope ofthe muted near end signal, and otherwise is the envelope of thenon-muted near end signal.
 2. A detection apparatus as claimed in claim1, effective in use to generate a detection signal, indicative of saidcondition, from the long term envelope of signal representing the resultof said subtraction, and the short term envelope of that signal.
 3. Adetection apparatus as claimed in claim 2, wherein the detection signalis representative of a condition where no far end speech is presentwhen, if a condition of detection of far end speech prevails, a shortterm envelope of said signal representing the result of said subtractionis less than a mean based on the product of a noise long term envelopeand the long term envelope of signal representing the result of saidsubtraction, and said detection signal is representative of a conditionwhere far end speech is present when, if a condition of detection of farend speech does not prevail, a very short term envelope of said signalrepresenting the result of said subtraction is greater than said meanbased don the product of a noise long term envelope and the long termenvelope of signal representing the result of said subtraction.
 4. Adetection apparatus as claimed in claim 2, wherein, in use, thedetection signal is applied to incrementally adjust the magnitude of ahold signal, said hold signal, when it reaches a threshold level,controls the telephone apparatus to effect said muting of the near endsignal, but otherwise which so controls the telephone apparatus that thenear end signal is not muted.
 5. A detection apparatus as claimed inclaim 1, having a near very short term envelope generating a near veryshort term envelope signal representing the very short term envelope ofthe near end sound; and a subtracter for generating a difference signalby subtracting a proportion of a muted component of the near very shortterm envelope signal from the far end signal, in case of existence of adetected condition of far end speech, or otherwise subtracting aproportion of a non-muted component of the near very short term envelopesignal from the far end signal; an output of the apparatus in use beingindicative of said condition of the existence of far end speech whensaid difference signal exceeds a predetermined magnitude.
 6. Apparatusas claimed in claim 5, wherein, in use steps of generating near veryshort term envelope signal, and generating said difference signal areexecuted repetitively.
 7. A method of detecting far end speech in atelephone apparatus having a first transducer for reproducing far endsound from a far end signal, when received by the telephone apparatus, asecond transducer for generating a near end signal from near end sound,for transmission from the telephone apparatus, and in which the near endsignal is muted for transmission from the telephone apparatus in thecase wherein it is intended that a user of the telephone apparatusshould listen to the reproduced far end sound, and a hybrid for passingthe near and far end signals from an external two wire link to and fromthe first and second transducers on respective first and second paths ofa four wire link, the method comprising effecting said muting of thenear end signal when signal derived from the said first path of saidfour wire link is detected as being representative of a condition ofexistence of a far end signal, and subtracting, from the signal derivedfrom said first path of said four wire link, signal determined asrepresenting sidetone signal deriving from the near end signal from thesecond transducer and appearing in the signal on said first path of saidfour wire link pursuant to traverse of at least part of the near endsignal through the hybrid, existence of said condition being determinedon the basis of the result of the subtraction of the signal determinedas representing sidetone signal from the signal on said first path ofsaid four wire link; wherein said subtracting is effected by generating,from the signal on said second path of said four wire link, a firstrelatively fast tracking envelope of the signal on said second path ofsaid four wire link, and subtracting a proportion of the first envelopefrom the signal derived from the first path of the four wire link; andwherein the first envelope is, in the case where a condition ofexistence of far end speech was previously detected, the envelope of themuted near end signal, and otherwise is the envelope of the non-mutednear end signal.
 8. A method as claimed in claim 7, further comprisinggenerating a detection signal, indicative of said condition, from thelong term envelope of signal representing the result of saidsubtraction, and the short term envelope of that signal.
 9. A method asclaimed in claim 8, wherein the detection signal is representative of acondition where no far end speech is present when, if a condition ofdetection of far end speech prevails, a short term envelope of saidsignal representing the result of said subtraction is less than a meanbased on the product of a noise long term envelope and the long termenvelope of signal representing the result of said subtraction, and saiddetection signal is representative of a condition where far end speechis present when, if a condition of detection of far end speech does notprevail, a very short term envelope of said signal representing theresult of said subtraction is greater than said mean based don theproduct of a noise long term envelope and the long term envelope ofsignal representing the result of said subtraction.
 10. A method asclaimed in claim 8, wherein the detection signal is applied toincrementally adjust the magnitude of a hold signal, which hold signalwhen it reaches a threshold level controls the telephone apparatus toeffect said muting of the near end signal, but otherwise which socontrols the telephone apparatus that the near end signal is not muted.11. A method as claimed in claim 7, further comprising generating a nearvery short term envelope signal representing the very short termenvelope of the near end sound; generating a difference signal bysubtracting a proportion of a muted component of the near very shortterm envelope signal from the far end signal, in case of existence of adetected condition of far end speech, or otherwise subtracting aproportion of a non-muted component of the near very short term envelopesignal form the far end signal; and generating an output indicative ofsaid condition of the existence of far end speech when said differencesignal exceeds a predetermined magnitude.
 12. A method as claimed inclaim 11, wherein steps of generating near very short term envelopesignal, and generating said difference signal are executed repetitively.